Several reasons spring to mind.
Some PCs don't have the STR/DEX to use certain weapons, so they use similar ones instead. One of my players has a PC with a low DEX who picked up Self Bow not Composite Bow.
Some weapons are a lot more expensive than others. Some are heavier than others. Where money is short or ENC is important, these factors come into play.
Some weapons are cultural. Lunars use Scimitars, Lunar enemies don't (in Glorantha). Vikings use Axes/Swords/Spears, Roman Legionaries don't use Axes, British Officers in the Zulu Wars wouldn't use a Spear and Shield.
Some weapons have religious merit or connotation and may be favoured or not favoured by members of certain cults. So, Humakti use Swords, Babeester Gori use Axes, Thuggees use Garrottes and so on.
Some weapons just aren't available. I've never been of the opinion that all weapons should be available everywhere. So, a phased plasma rifle might be in the rulebook, but that doesn't mean you should be able to pick one up anywhere. Similarly, composite bows should only really be available in areas with contact with those cultures that use composite bows.
Some weapons are banned. Most Gloranthan cities ban the carrying of long weapons. Imagine walking through modern day London with a sub-machine gun (OK, Los Angeles shouldn't be a problem, though
). If certain weapons were banned, it might slow down their takeup.
Some weapons are more suitable for certain situations. A thief, for example, might carry a knife rather than a dagger as it is less conspicuous, doesn't make him look like a killer and can be used to cut bread.